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Astro 358/Spring 2009! (48540)!
Galaxies and the Universe !
Figures for Lecture 3-5 (Th Jan 29 to Tu Feb 7)
Do external galaxies exist ? !
1) 1920s : The Great Debate": Harlow Shapley vs Heber Curtis !
Are spiral nebulae large distant systems like the Milky Way or ! nearby small objects associated with our own Galaxy?!
Shapley: Milky Way is huge (diameter ~100 kpc, Sun off center) and spiral nebulae ! are small nearby companions !
Curtis: Milky Way is small (d~18 kpc, Sun at center) and spiral nebulae are external ! stellar systems (galaxies) comparable to the Milky Way, but very distant from us!
2) Progress: Accurate distances + realization that galaxies have dust!
3) 1927: Edwin Hubble use Cepheids to show M31 distance = 770 kpc !! ! !M31 and other spiral nebulae are external galaxies!
Do external galaxies exist ? !
Today we know : over a billion galaxies exist beyond our Milky Way!
There are 2 fundamental stellar components of galaxies !
a) Flattened component = stellar disk ! b) Puffed-up Spherical/Spheroidal/Triaxial (SST) stellar components ! (e.g, classical bulges, massive Ellipticals)!
These 2 components differ fundamentally in ! ---> 3-D structure and ratio Vc/ of ordered to random motion! ---> formation pathways!
See in class notes!
Fundamental Stellar components of Galaxies !Stellar disks and Spherical/Spheroidal/Triaxial components!
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Galaxies types based on total mass and morphology!
1) High mass! - Elliptical galaxies (E, cE)!- Lenticular galaxies (S0) !- Spiral galaxies (Sa to Sd)!
2) Low mass!- Irregular galaxies (Sm, Im Irr)!- Dwarf galaxies (dIrr, dE, dSph)!
3) Mixed !-Peculiar/Interacting !
See in class notes!
Galaxies types based on total mass and morphology!
Properties of E, Spirals, Irregulars and Dwarfs!
Table 1. Properties of E, Spirals (Sa-Sd), Irregulars (Sm,Im,Irr) & Dwarfs (dIrr,dE, dSph)
E Sa to Sc Sd/Sm Im/Irr Dwarfs (dE,dIrr,dSph +BCD)
Dynamical Mass Md (M!) 108 ! 1013 109 ! 1012 108 ! 1010 108 ! 1010 107 ! 109
D25 (kpc) 1-200 5-100 0.5-50 0.5-50 dE: 1-10
dIrr: 1-10
dSph: 0.1-0.5
BCD: < 3
MB -15 to -23 Sa: -17 to -23 -15 to -20 -13 to -18 dIrr: -15 to -18
(mostly -17 to -23) Sb: -17 to -23 BCD: -14 to -17
-15 to -19 for cE Sc: -16 to -22 dE: -13 to -19
-19 to -23 for rest dSph: -1.5 to -15
LB (L!) (0.3 ! 4) " 1010 for Sa: (1 ! 3) " 1010 (< 0.1 ! 2) " 1010 · · · · · ·
intermediate and giant E
but lower for low
luminosity E
Mass-to-light ratio 10 - 100 Sa: 6.2 # 1 # 1 dSph: # 5 to # 3500
Md/LB (M!/L!) Sb: 4.5
Sc: 2.6
B - V 0.7-1.2 Sa: 0.75 0.47 0.37 · · ·
Sb: 0.64
Sc: 0.52
Mgas/M" · · · Sa: 0.04 0.25 · · · dIrr: > 0.25
Sb: 0.08
Sc: 0.16
M(HI)/LB < 0.1 Sa: 0.05 - 0.1 0.25 to > 1 · · · · · ·
Peak Vc (km s#1) 0-100 Sa: 163-367 · · · 50-70 · · ·
Sb: 144-330
Sc: 99-304
Properties of E, Spirals, Irregulars and Dwarfs!
Table 1—Continued
E Sa to Sc Sd/Sm Im/Irr Dwarfs (dE,dIrr,dSph +BCD)
Velocity dispersion 50 - 400 5 - 50 ! 10 ! 10 dSph: 3.5 " 20
#!!$ (km s"1)
Vc/!! 10 - 40 · · · ! 5 · · ·
Stars dominating color G/K · · · · · · · · · · · ·
Note. — References: Tables 3.1/EAC, 3.2/EAC, 5.1/GU. dSph data references from 1998,
ARA&A, 36, 435 and 2011, ApJ, 733, 46.
Terms used in table refer to the following:
D25 = Diameter where B-band surface brightness reaches 25 mag arcsec"2
MB = Absolute blue magnitude
LB = Blue luminosity
Md = dynamical mass = mass of visible matter (gas+stars) + mass of dark matter
M! = mass of stars
Mgas = mass of atomic and molecular gas
M(HI) = mass of atomic hydrogen
Vc = peak circular speed of stars
!!= velocity dispersion of stars
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Giant elliptical M87!
Elliptical Galaxies Spiral Galaxies!Spirals have extended outer stellar disk with spiral arms and central stellar bulge!
Spiral Galaxies: Edge-on view!
NGC 4594 or M104 (Sombrero) An unusual spiral, with an unusually large bulge and a dusty disk, seen edge-on (Bulge/Disk luminosity ratio > 2 )
Typical spiral, where outer disk is more massive and luminous than central bulge (Bulge/Disk luminosity ratio well below 1) A stellar bar drives gas (via shocks+ torques) from the disk into the central kpc where !
it ignites powerful central bursts of star formation:.. 10 billions Lsun!! It may help to feed the central monster (black hole) !
Strongly Barred spiral (SBbc) NGC 1300
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Weakly barred and unbarred spirals!
Unbarred spiral (SAab) NGC 4622 Weaky barred spiral (SABc) NGC 674 !
Strongly Barred (SB) spirals SBa SBb SBc
Unbarred (SA) spirals SAa SAb SAc
Spirals divided into Hubble type Sa to Sd!
Milky Way = a barred (SBbc) çgalaxy
Face-on view (Artist’s conception)
Edge-on view (Artist’s conception) Qualitatively ok but missing stellar bar and wrong shape for bulge
Our Milky Way !Properties of Spiral (Sa to Sd) & Irregular (Irr, Sm, Im) galaxies!
Table 3.2, EAC
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NGC 5866
NGC 4382
NGC 2787
NGC 4417
NGC 4578
Lenticular (S0) galaxies!
Moderately inclined S0S: Is outer structure really ! a disk?!
Edge-on and highly inclined SO: outer disk! is clearly visible!
NGC 4382
NGC 4578
Moderately inclined SOs!
NGC 4406
NGC 4472
Elliptical galaxies !
Hard to separate moderately inclined unbarred S0s from Es!
Some S0s are barred and must thus have outer disks!
Barred S0 galaxies in the Coma cluster!(Credit Marinova, Jogee, & Coma ACS Team, 2012, ApJ, in press)!
Since bars only form in disks, the presence of barred S0s implies that at least !some S0s harbor extended outer disks !
Why are S0s important?
Morphology-density relation: the frequency of (E+S0) galaxies relative to spiral galaxies is much higher in high density environments (e.g., galaxy clusters ) than in low density environments (e.g., field)
Frequency of E+S0:Sp = 40%+50% :10% in cluster = 10%+10%: 80% in field
Core of Abell 1689 cluster
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There is a (fairly smooth) trend of higher luminosity and redder colors from Sa-Sc to S0s
Transition in color and luminosity from S0 to Scs!
Large Magellanic Cloud or LMC Type (SB(s)m) Distance = 50 kpc Image diameter = 10 kpc
Small Magellanic Cloud or SMC Type = SB(s)m pec or Im Distance = 63 kpc Image diameter = 6 kpc
Irregular Galaxies (Sm, Im, Irr) !
Low mass, gas-rich; Irregular visual morphology ; Stellar (Vc/ ) > 1
Leo I (dSph), satellite of M Way
Dwarf Galaxies (dIrr, dSph, dEs)!
NGC 1705 (dIrr) !(Credit: NASA, ESA, Hubble Heritage)! NGC 205 (dE) !
(Credit: NASA, ESA, Hubble Heritage)!
Some dEs show disk features (bar, spiral arms)!
(Barazza et al .2002, A&A)!
Specially processed images (unsharp masked images) of bright dE and dS0s reveal:!
- IC3468 (dE) ; short bar in a nearly face-on disk or small disk seen edge-on in a spheroid !
- IC3349 (dE) ; show a weak bar-like elongated central structure!
- NGC 4431 (dS0) ; strong long ! bar!- IC0783 dS0); spirals !
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- dSph low blue luminosities (overlapping with those of globular clusters)! very low mean surface brightness (25-27 mag arcsec-2 in B) visible only in the local Group!
- (dSph + bright dEs) Half-light radius re rises only slowly with luminosity causing their mean ! ! surface brightness to rise with luminosity. !
(Bright Es and bulges) Half-light radius re rises steeply with luminosity, causing their mean !! ! surface brightness to fall at higher luminosities !
Difference between (dSh + bright dEs) and (bright E+bulges)!
dSph
Bright dEs
Es + bulges
dSph
Bright dEs
Es + bulges
Polar ring galaxy NGC 4650
Ring galaxy AM 0644-741 50,000 ly across
Cartwheel galaxy Head-on collision
Peculiar/Interacting Galaxies!
NGC 4736 / The Mice (Credit: NASA/STScI/Hubble Heritage!
HST image shows details of a collision between 2 spiral galaxies 30 kpc apart.! Example of a nearby ongoing major merger!
Example of ongoing major merger!
The Antennae system is believed to be an ongoing major merger of 2 spirals The HST image of the central region of the merger: shows stripped gas and dust lying between the 2 disks
Example of ongoing major merger!
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(Mihos & Hernquist; DM halo + Stars =yellow, gas= blue, Duration = 1 Gyr)
distance =
Simulation of a major merger between 2 spirals!
Major mergers: gas inflows, conversion of gas to stars, violent relaxation of stars converting disk configuration into spheroidal distribution of stars
Data The Toomre
Sequence
The Elliptical/Peculiar galaxy (NGC 1316) ! recently accreted!small gas-rich galaxies!
Example of a minor merger!
2 fundamental groups of galaxies dominated by disks versus puffed-up
SST components!
SST = Spherical/Spheroidal/Triaxial components!
Massive Elliptical
Can divide galaxies into 2 groups where the stellar mass is dominated! By puffed-up SST ! !versus By disks! (supported by velocity dispersion) !(supported by rotation) ! - Massive Es ! ! ! ! - Most spiral galaxies , Irrs, dIrr! - Some S0s + rare spirals dominated by - Some S0s dominated by outer disks! classical bulges ! ! ! ! - Under debate: Some dE, low mass E !
2 fundamental groups of galaxies dominated !by disks versus puffed-up SST components!
Spirals
Irr (LMC)
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In plane of central SB vs absolute blue magnitude (Es + bulges of spirals) follow a radically different relation from (dIrr, dE, and disk of spirals). Do the two groups represent! - galaxies dominated by disks vs puffed-up SST components! - different sources of support from gravity (random velocity of stars vs ordered rotation or stars )! - different formation pathways? !
Difference between !(dIrr + dEs + disk of spirals)
versus !(bright Es+ bulges)!
(Mihos & Hernquist; DM halo + Stars =yellow, gas= blue, Duration = 1 Gyr)
distance =
Simulation of a major merger!
Major mergers: gas inflows, conversion of gas to stars, violent relaxation of stars converting disk configuration into spheroidal distribution of stars
Data The Toomre
Sequence
Building a spiral galaxy (with a classical bulge and disk) starting from mini proto-disk galaxies over 13 billion years!
Caroon movie (courtesy: Frank Governato)!Galaxy grows via smooth accretion, minor mergers and major mergers!
Ideally, we want to measure ! R = (Stellar mass in Spherical/Spheroidal/Triaxial components / Stellar mass in disk)! = (M* in SST)/(M* in Disk) ! = (M* in classical bulge or massive Elliptical )/ (M* in disk)!
In practice, we measure in (spiral, S0s, E), the bulge-to-disk (B/D) ratio ! B/D = (M* in bulge)/ (M* in disk) ! = 1 for massive E, 0 for pure disk galaxy, intermediate for Spirals and S0 !!where bulge is defined as central component containing excess light over outer disk!
B/D ratio as a proxy for Ratio of stellar mass in (SST/Disk)!
One problem in using B/D as a proxy for R is that “bulges” defined in the above way include not just puffed-up classical bulges, but also disky bulges, peanut bulges !
SAa SAb SAc <-------------------------------------------------------
B/D ratio rises from Sc to Sa
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Galaxy Classification Schemes!
Hubble-Sandage Classification System (1961) extended from Hubble (1936)
------------>!Ellipticals E0 to E7 !have rising ellipticity!e =1-b/a = 0 to 0.75!
--------------------------------->!Spirals from types Sa to Sc !have (lower B/D, less smooth disk, more open spiral arms).!Letter B denotes Barred!
S0!
Limitations of !Hubble-Sandage!
classification scheme!
1) Assumes B/D, smoothness, ! of disk and tightness of spiral ! arms are correlated. ! Where does a spiral with low ! B/D and smooth disk fit in?!
2) Low mass (Sd, Irr, Dwarf) and Pec/Interacting galaxies do not fit in!
3) Weakly barred galaxies do not! fit in !4) Based on optical images. But ! need multi-wavelength images ! and kinematics for a more ! complete view!
De Vaucouleurs's extension of the Hubble Classfication system (1959)
a) added low mass galaxies (Sd, Sm, Im) , "Pec" notation,! b) added bar strength : SB= strongly barred, SAB=weakly barred, SA=unbarred!c) added dimensions for ring (r) and spiral arms (s)!d) fine-tuned transition from E to S0 as (E+ S0- S0+} !
Example: NGC 2782 is typed as SAB(rs)a in !the RC3 catalog:!
SAB(rs) means it is ! a weakly barred spiral, ! with a ring and spiral arm! Sa means it has high !B/D, tight and smooth spiral arms!